Recovery of zinc



Patented Feb. 15, 1944 RECOVERY OF ZINC William A. Ogg, Huntington, W. Va.

No Drawing. Application December 22, 1941, Serial No. 423,951

6 Claims.

This invention relates to the recovery of zinc from its ores and the object is to provide a method utilizing a novel composition of matter of mixture of ingredients providing with the use oi comparable equipment an increased output as compared with existing smelting practice. The method is capable of practice with existing smelting furnaces greatly to increase the productivity of the same. Such increase of productivity is, of course, of economic advantage and at the time of this writing is of vital national significance independent of considerations of operating profit.

My invention will be well understood by reference to the following description. Since the mechanical equipment referred to in the course of the description is well known to zinc metallurgists, it is considered unnecessary to append drawings to this specification.

The smelting of zinc, chemically considered, is an endothermic reaction involving the reduction of zinc oxide, in the form of roasted ore, by carbon, usually in the form of coke breeze, resulting from heating indirectly a mixture of the solid reagents in powdered form. The most generally practised process of smelting zinc utilizes a so-called Belgian retort in which the mixture of roasted zinc ore and carbonaceous material is charged into refractory retorts, generally of cylindrical shape, to the outside of which heat is applied, usually for a period of twenty-four hours when the charge is considered worked off and the remaining residue removed from the retort and a fresh mixture charged. The amount of carbonaceous material used varies from about 30 to 60% of the weight of the roasted ore. I have demonstrated that substantially complete reduction of zinc may be eflected by mixing with the roasted ore or sinter about 15% by weight of coke breeze, which for practical purposes may be considered substantially the theoretical proportions required for'the reaction. The excess used for various practical reasons, among which may be mentioned (1) to keep the charge open so that the gases and vapors generated may escape easily; (2) to prevent the contacting of slag-forming materials which might unite and fuse; (3) to absorb any slag formed and thus prevent the slag from entrapping zinc and also from reaching and corroding the retort wall; and (4) to fill up the retort and thus provide solid material to conduct the heat through the charge.

The evolution of zinc vapor during the course 'over the theoretical requirements of carbon is of the process is not at a constant rate and while theoretically the heating might be continued until practically the entire zinc content was recovered, this is not economically feasible and on a twenty-four hour cycle as described a recovery .of 90% of the available zinc is considered good practice. The length of time required for such recovery is due to the fact that the thermal conductivity of the roasted ore and the carbonaceous material used is so poor that the rate of heat transfer is exceedingly slow. This has been demonstrated in practice by attempts to use retorts formed of silicon carbide which were found to be of little advantage because of the low conductivity of the charge. The thermal conductivity of the charge itself also limits the size of the retort usedin order to permit an economic recovery of zinc within a given time. Hence as a practical matter with a twenty-four hour cycle the diameter of the retort cannot be increased beyond a certain limit, about nine inches inside.

In accordance with my invention I reduce the amount of carbonaceous material of low heat conductivity which is used to an amount approaching that theoretically required for the chemical reaction, and replace the balance of the volume previously used with some material of high heat conductivity (preferably of low specific heat) which is inert in the sense that under the atmospheric and temperature conditions of the reaction-chamber it does not substantially, enter into reaction with the other constituents of the charge and which is preierabiy of such a character as to resist any substantial physical transformation during the treatment whereby it may be recovered for re-use.

Thus to mention a specific example, for use with Joplin ore which has a hint gangue, silicon carbide in granular form is well adapted for the novel ingredient. This material in itself has a thermal conductivity of about 9 as contrasted with a conductivity for the zinc sinter of about 1 and for the coke breeze of about These figures are relative andI am not aware of experimental data indicating thermal conductivity for the materials speclfiedin the loose or granular condition in which they are used in the retort but the relative efficiency as regards cohduction of heat is indicated by the figures given.

To consider a specific example, we may take as a unit a so-called Belgian retort, the interior chamber of which is a cylinder 4 /2 feet in length and of 8 /2 inches inside diameter, that is, 1.78

cubic feet. A typical charge under present practice for such a retort would be pounds of zinc sinter and 40% by weight of coke breeze, that is, 84 pounds, substantially filling the retort. In accordance with my invention for the same amount of sinter I may use pounds of coke breeze, representing a little less than 18% by weight of the sinter and being close to the theoretical amount of carbon required to reduce the zinc oxide present. The balance of the retort is filled with granular silicon carbide crystals. This requires about 95 pounds, the greatly increased weight being due to the comparatively great den.- sity of the silicon carbide. Computing theoretically from available figures for heat conductivity, the heat conductivity of the second charge as a whole as contrasted with the first has been increased over 100%. Because of the relatively great weight of silicon carbide the heat requirements per pound of zinc reduced have been somewhat increased but not enough to detract from the great advantages obtained by the increase in conductivity.

In referring to the increase of heat conductivity above mentioned I have considered only an average based on the proportions of the ingredients in the original charge. However, it is to be noted that during the reaction a certain amount of the roasted ore and coke disappears and in prior practice what remains behind becomes less and less conductive on account of the constantly growing percentage of coke breeze which has a very poor thermal conductivity, and for this reason the rate of production of zinc in practice falls off and for economic reasons is interrupted before the reaction is complete. In accordance with the process just described wherein the amount of coke breeze is reduced and silicon carbide substituted, instead of the remaining charge becoming poorer in thermal conductivity, it is constantly improving because of the increasing percentage of silicon carbide. I

Eliminating for the moment other economic factors and considering for simplicity a doubling of the rate of heat application, it will be seen that for the same percentage return the furnace may be operated on a twelve hour cycle instead of a twenty-four hour cycle with a doubling of its output.

Silicon carbide is of itself, of course, expensive and to be practical economically it must be recovered and reused. This may readily be done.

an inch mesh. The zinc sinter may be crushed to pass through the V8 inch mesh and mixed with the coke breeze and silicon carbide. When the residue is discharged it is passed over a A; inch screen and there remains on the screen all pieces over A; of an inch in size, which include substantially all the silicon carbide crystals and the pieces of coke breeze larger than A; of an inch in size. What passed through the 4; inch screen is almost worthless and can be discarded. The material retained on the screen is easily separated because of the wide disparity in specific gravity between the coke and the silicon carbide, recovering the silicon carbide and the usable coke separately for re-use.

An alternative application of the process contemplates the increase in the amount of zinc charged. Thus by way of example considering a retort of the same dimensions we may increase the zinc content one-third, charging 113. pounds of zinc sinter. The same proportion of coke breeze as in the example previously given would be 20 pounds and about "37 /2 pounds of silicon carbide crystals would then fill the retort. As contrasted with the conventional sinter and coke mixture, the theoretical increase in thermal conductivity in this mixture is about 60% and moreover the heat requirements per pound of zinc reduced also show a slight decrease. As contrasted with the previous example of the invention, the rate of reduction is decreased. Operating the furnace in about the same manner as customary, it would not be possible to charge the retorts twice daily, but, on the other hand, since the zinc charge has been increased one-third, the cost of smelting is decreased substantially in direct proportion to the increase in charge. Moreover, due to the greater heat conductivity of the charge, while the zinc content is increased only one-third, the available heat is increased in a greater percentage. Therefore, if the operation is continued for twenty-four hours, the additional heat available under conditions involving, as above described, a constantly improving heat conductivity will permit the reaction to be pushed more nearly toward completion within the time available with a higher percentage recovery of zinc.

Obviously. various factors including prices, profit margins, labor charges, time convenience and the like, all have to be considered in determining the optimum combination of charge and recovery which will give the best results. I

Among these factors it may be mentioned that when an almost complete reduction of zinc may be efiected there will be only a very small amount of blue powder, etc., to be resmelted. Under ordinary circumstances in addition to the blue powder a certain'ambunt of zinc always remains in the residue andfi s recovered by lies and recharged to the retorts. Where the recovery of zinc, however, is practically complete, it is uneconomic to try to recover the small remaining amount of zinc by jigs and the retort volume formerly required for resmelting this recoverable material is'available for additional charge, thus increasing the smelting capacity of the furnace.

In the above-discussion I have considered a typical retort as now used and a great advantage of my invention is that it may be practised with existing equipment without any changes involving capital expenditure. However, advantage may also be taken of the increased thermal conductivity of the charge as provided under the invention to increase the capacity of a furnace installation by enlarging the diameter of the retort so as to enable a greater charge to be made.

The use of retorts themselves made of silicon carbide has not hitherto been advantageous because of the oor conductivity-of the charge as hitherto used, but they may advantageously be utilized with the highly conductive charge in ac,- cordance with the present invention.

As another example of high heat conductive material which may be utilized under some conditions I may mention steel shot, that is, steel particles conveniently in the form of small ductivity of a charge mixture utilizing steel shot is comparable to that obtained in the case of silicon carbide although the heat requirements per pound of zinc are greater, and indeed somewhat greater than in the case of the normal charge although not enough to countervail the advantages arising from greatly increased heat conductivity. The shot may be recovered from the residue for re-use by magnetic separation.

As a further example of inert high heat-conductive material I may cite fused alumina (aluminum oxide).

Another smelting process used to some extent in the United States involves the briquetting of the charge, the briquets then being heated in a vertical shaft furnace. Silicon carbide or other material of high thermal heat conductivity may be incorporated in such briquets in place of a portion of the carbon customarily used in the same manner as in the case of the relatively uncompacted charge of the Belgian retorts, although the recovery of the material from the spent charge is somewhat more difficult.

It is impossible to catalog the inert materials which might be used in the practice of the present invention, particularly since economic considerations in a particular place or time might render one material available which under other conditions it would not be particularly advantageous to use.' The skilled metallurgist Will reject materials obviously inapplicable to the process and in the case of other materials a simple test not in itself of an inventive nature will permit selection.

I am aware that the invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof, and I therefore desire the present embodiment to be considered in all respects as illustrative and not restrictive, .as is in fact clear in several matters from the description itself. Reference is to be had to the appended claims to indicate those principles of the invention exemplified by the particular embodiment described and which I desire to secure by Letters Patent.

I claim:

1. In the smelting of zinc the method which comprises the step of preparing a smelting charge by mixing with the crude oxide carbon in proportion not greatly exceeding theoretical combining proportion and mixing therewith inert material of high heat conductivity to provide a greatly increased conductivity of the charge as compared with a similar charge wherein a like volume of carbon replaced the inert material and disengaging the zinc from said charge by indirect heating in a retort.

2. In the smelting of zinc the method which comprises the step of preparing a smelting charge by mixing with crude oxide and reduction material granular material of high heat conductivity in amount to increase substantially the average heat conductivity and disengaging the zinc from said charge by indirect heating in a retort.

3. A method of promoting the endothermic reaction between zinc ores and reduction material for the recovery of zinc which comprises mixing with said materials in granular form grains of material of relatively great heat conductivity and which are substantially inalterable under the atmospheric and temperature conditions existing in the reaction vessel to provide a mixture having as a whole a heat conductivity substantially higher than a like mixture wherein a like volume of reduction material replaced the inert material and then disengaging the zinc from said charge by indirect heating in a retort.

4. In the smelting of zinc the method which comprises increasing the average thermal conductivity of the charge of crude oxide and reduction material to be loaded into the customary retorts by mixing therewith inert granular material of high heat conductivity and then disengaging the zinc from said charge by indirect heating in a retort.

5. In the smelting of zinc the step which comprises loading the customary retort with crude zinc oxide and carbonaceous material with which is mixed substantial amounts of granular silicon carbide markedly increasing the thermal conductivity of the charge.

6. As a new composition of matter for the production of zinc by indirect heating, crude 

